Posted by gosolar23 on October 18, 2005, 4:06 pm
I live in MILD Seattle. Today's high is 63 degrees and sunny. I have
a quote from Energie-Solaire for an UNGLAZED collector that would cover
my 550 ft shed dormer.
They suggest an unglazed, stainless steel collector that functions as a
roof. I want to put in a large volume storage (5000 gallons) with
large amount of insulation.
I have heard response from some people that this is a bad idea. Do you
have any helpful thoughts? I would apreciate them.
I don't think these Swiss engineers are nincompoops. They seem like
they understand everything, but did they forget the second law of
thermodynamics or something?
Posted by Robert Scott on October 18, 2005, 6:27 pm
Well, an unglazed collector is certainly less efficient than a glazed
one, but it may still be more cost-effective. Just think how much
cheaper it is to make an unglazed collector.
Posted by gosolar23 on October 18, 2005, 8:06 pm
And you are 20% moore efficient without the glazing to reflect energy.
Posted by daestrom on October 18, 2005, 9:15 pm
Well, a layer of typical glass will reduce convection losses, that is true.
But it also blocks some of the sunlight. One source...
Says a single layer will only let 84% of the solar energy through.
So, it becomes a question of how hot the water/collector surface get
compared to *your* local ambient air temperature, vs. how much energy is
blocked by glazing.
Nick could probably give you details, but if it's 63F on a 'typical' warm
day with the sun shining and your water temp is 120F (for a delta T of
~57F), an air film on the surface with no significant breeze would lose
about 57*(0.61) = ~35 BTU/hr per ft^2.
One layer of glazing would block about ~47 BTU/hr per ft^2.
So, it seems to be a bit of even-steven. Mind you, the heat losses with
unglazed will go up sharply in a windy situation (or colder outdoor
But if this day seems 'typical', you can get comparable performance with a
lot lower cost.
(mind you, this hasn't considered the 'green-house' effect of trapping IR
inside the glazing, but selective coatings can mitigate that loss)
Posted by Rob Dekker on October 19, 2005, 3:34 am
Thanks for the calculations daestrom !
What if there is little or no insulation in the back of the panel (to reduce
Does the same convection loss apply there too ?
So would he thus loose 70Btu/hr on a wind-still day ?
And what if there is air gap (for moisture/ventilation) between the roof and and
the collector ?
I would guess that air thermo-syphoning would create a natural air flow in that
adding greatly to the heat losses... Does it ?
But it also blocks some of the sunlight. One
to *your* local ambient air temperature, vs. how
with the sun shining and your water temp is 120F
would lose about 57*(0.61) = ~35 BTU/hr per ft^2.
unglazed will go up sharply in a windy situation (or
inside the glazing, but selective coatings can mitigate